Reduction in Potency of Floral Scents as a Result of Increasing Pollutant Concentrations

Flowers release blends of molecules whose characteristic scents attract insect pollinators. During their atmospheric transport, scents react with oxidants such as ozone, hydroxyl radical, and nitrate radical. As a result, in polluted air masses, ambient scent concentrations and chemical composition of the original blends can change with distance from sources, thereby potentially impacting the foraging patterns of pollinators. Bouquets of floral scents with different reactivity are examined in this study to estimate chemical degradation and changes in composition of floral odor plumes downwind from sources. Reactions of ozone with the floral scents belonging to monoterpenes and sesquiterpenes generate high yields (> 0.5) of hydroxyl radical. Thus, this study additionally evaluates the hypothesis that in ozone-rich environments the scents are more rapidly destroyed due to the concomitantly enhanced hydroxyl radical formation. Numerical model simulations, employing single-particle Lagrangian and Eulerian models, are carried out to determine the floral scent plume characteristics under the influences of increasing levels of atmospheric oxidants, nitric oxide, and turbulence. Averaged scent concentrations are computed as a function of distance away from sources to determine the extent to which ambient air pollutants chemically degrade and modify the original composition of floral fragrance blends. Results indicate that oxidants substantially modify the bouquet of reactive floral scents. In polluted air masses, with ozone levels of 120 parts per billion, scent abundances away from sources markedly decrease in response to rapid chemical reactions, with the most reactive odors reaching 10 - 25% of the original abundance within downwind distances of 50 - 100 m from sources. Even moderate levels of oxidants (e.g., ozone mixing ratios of 60 parts per billion) cause appreciable reductions in the downwind footprint of floral scents. Because floral scents are essential for pollinators to locate flowers, the results of this study suggest that air pollutants may need to be considered as a potential disruption of pollination.